diff --git a/extensions/openpower-pels/ascii_string.cpp b/extensions/openpower-pels/ascii_string.cpp
index 7d4d4f3..7e0f493 100644
--- a/extensions/openpower-pels/ascii_string.cpp
+++ b/extensions/openpower-pels/ascii_string.cpp
@@ -51,7 +51,9 @@
     }
     else // BMC Error
     {
-        setByte(2, entry.subsystem);
+        // If subsystem wasn't specified, it should get set later in
+        // the SRC constructor.  Default to 'other' in case it doesn't.
+        setByte(2, entry.subsystem ? entry.subsystem.value() : 0x70);
     }
 
     // Then the reason code
diff --git a/extensions/openpower-pels/registry.cpp b/extensions/openpower-pels/registry.cpp
index 2c66e0f..b853bca 100644
--- a/extensions/openpower-pels/registry.cpp
+++ b/extensions/openpower-pels/registry.cpp
@@ -610,7 +610,11 @@
         {
             Entry entry;
             entry.name = (*e)["Name"];
-            entry.subsystem = helper::getSubsystem((*e)["Subsystem"]);
+
+            if (e->contains("Subsystem"))
+            {
+                entry.subsystem = helper::getSubsystem((*e)["Subsystem"]);
+            }
 
             if (e->contains("ActionFlags"))
             {
diff --git a/extensions/openpower-pels/registry.hpp b/extensions/openpower-pels/registry.hpp
index 180090c..bb26c70 100644
--- a/extensions/openpower-pels/registry.hpp
+++ b/extensions/openpower-pels/registry.hpp
@@ -132,7 +132,7 @@
     /**
      * @brief The PEL subsystem field.
      */
-    uint8_t subsystem;
+    std::optional<uint8_t> subsystem;
 
     /**
      * @brief The optional PEL severity field.  If not specified, the PEL
diff --git a/extensions/openpower-pels/registry/README.md b/extensions/openpower-pels/registry/README.md
index 89d4578..ac0c266 100644
--- a/extensions/openpower-pels/registry/README.md
+++ b/extensions/openpower-pels/registry/README.md
@@ -46,12 +46,24 @@
 ### Subsystem
 This field is part of the PEL User Header section, and is used to specify
 the subsystem pertaining to the error.  It is an enumeration that maps to the
-actual PEL value.
+actual PEL value.  If the subsystem isn't known ahead of time, it can be passed
+in at the time of PEL creation using the 'PEL\_SUBSYSTEM' AdditionalData field.
+In this case, 'Subsystem' isn't required, though 'PossibleSubsystems' is.
 
 ```
 "Subsystem": "power_supply"
 ```
 
+### PossibleSubsystems
+This field is used by scripts that build documentation from the message
+registry to know which subsystems are possible for an error when it can't be
+hardcoded using the 'Subsystem' field.  It is mutually exclusive with the
+'Subsystem' field.
+
+```
+"PossibleSubsystems": ["memory", "processor"]
+```
+
 ### Severity
 This field is part of the PEL User Header section, and is used to specify
 the PEL severity.  It is an optional field, if it isn't specified, then the
diff --git a/extensions/openpower-pels/registry/schema/schema.json b/extensions/openpower-pels/registry/schema/schema.json
index 3bec95d..6fa6cf4 100644
--- a/extensions/openpower-pels/registry/schema/schema.json
+++ b/extensions/openpower-pels/registry/schema/schema.json
@@ -35,6 +35,8 @@
 
                     "Subsystem": {"$ref": "#/definitions/subsystem" },
 
+                    "PossibleSubsystems": {"$ref": "#/definitions/possibleSubsystems" },
+
                     "Severity": {"$ref": "#/definitions/severity" },
 
                     "MfgSeverity": {"$ref": "#/definitions/mfgSeverity" },
@@ -56,13 +58,23 @@
                     "Callouts": {"$ref": "#/definitions/callouts"}
                 },
 
-                "required": ["Name", "SRC", "Subsystem", "Documentation"],
+                "required": ["Name", "SRC", "Documentation"],
                 "additionalProperties": false,
 
                 "not":
                 {
                     "required": ["CalloutsUsingAD", "Callouts"]
-                }
+                },
+
+                "oneOf":
+                [
+                    {
+                        "required": ["Subsystem"]
+                    },
+                    {
+                        "required": ["PossibleSubsystems"]
+                    }
+                ]
             }
         },
 
@@ -195,6 +207,18 @@
                      "user_error", "corrosion"]
         },
 
+        "possibleSubsystems":
+        {
+            "description": "Required when the PEL creator uses PEL_SUBSYSTEM in the AdditionalData property to pass in the subsystem.  Used by scripts that generate documentation to build all possible SRC ASCII strings for this error.",
+            "type": "array",
+            "items":
+            {
+                "$ref": "#/definitions/subsystem"
+            },
+            "minItems": 1,
+            "uniqueItems": true
+        },
+
         "systemAndSeverity":
         {
             "description": "A severity entry that has an optional system type qualifier.  Used when the severity needs to be based on the system type.",
diff --git a/extensions/openpower-pels/user_header.cpp b/extensions/openpower-pels/user_header.cpp
index 7aed654..b2365a9 100644
--- a/extensions/openpower-pels/user_header.cpp
+++ b/extensions/openpower-pels/user_header.cpp
@@ -58,26 +58,44 @@
     _header.subType = 0;
     _header.componentID = static_cast<uint16_t>(ComponentID::phosphorLogging);
 
-    _eventSubsystem = entry.subsystem;
+    std::optional<uint8_t> subsys;
 
     // Check for additional data - PEL_SUBSYSTEM
     auto ss = additionalData.getValue("PEL_SUBSYSTEM");
     if (ss)
     {
         auto eventSubsystem = std::stoul(*ss, NULL, 16);
-        std::string subsystem =
+        std::string subsystemString =
             pv::getValue(eventSubsystem, pel_values::subsystemValues);
-        if (subsystem == "invalid")
+        if (subsystemString == "invalid")
         {
             log<level::WARNING>(
-                fmt::format("UH: Invalid SubSystem value:{:#X}", eventSubsystem)
+                fmt::format(
+                    "UH: Invalid SubSystem value in PEL_SUBSYSTEM: {:#X}",
+                    eventSubsystem)
                     .c_str());
         }
         else
         {
-            _eventSubsystem = eventSubsystem;
+            subsys = eventSubsystem;
         }
     }
+    else
+    {
+        subsys = entry.subsystem;
+    }
+
+    if (subsys)
+    {
+        _eventSubsystem = *subsys;
+    }
+    else
+    {
+        // Gotta use something, how about 'others'.
+        log<level::WARNING>(
+            "No PEL subystem value supplied for error, using 'others'");
+        _eventSubsystem = 0x70;
+    }
 
     _eventScope = entry.eventScope.value_or(
         static_cast<uint8_t>(EventScope::entirePlatform));